1. Field of the Invention
The present invention relates to a control system for a clothes dryer and, more particularly, to a clothes dryer including a heater control system configured such that the power supplied to the heater is altered in an efficient manner by cycling the amount of heat supplied by the heater between a high power level and a low power level during a drying operation.
2. Discussion of the Prior Art
It is well known in the art to provide a clothes dryer with a mechanism to control the amount of heat supplied during a drying process. For example, a clothes dryer having a simple time-dry control allows a user to place wet articles inside the dryer and to select both a duration and a temperature setting for the drying process. In such a process, drying simply continues until the set time expires. Typically, in such an operation, the dryer will initially set its electrical heating element to a full power of approximately 5,000 watts. The temperature within the dryer will then rise until it reaches a preset maximum. Thereafter, the dryer will enter a no power mode during which the heating element will be set to zero power. In general, the no power mode has been employed to avoid overheating the articles of clothing. The dryer will then alternate between full power and no power modes until the set time has expired.
In such an operation, the user has no control over what preset maximum power will be provided to the heater of the dryer. Further the amount of heat provided in the no power mode, namely zero power, is also not affected by either the user or the control circuit in prior art devices. This is particularly problematic for delicate clothing which can be easily damaged by high temperatures. To address this potential problem regarding delicate clothing, dryers typically allow for a fluff cycle during which time the heating element is not activated at all. Of course, without any heating, drying times can be excessively long.
It is also well known in the art to provide a clothes dryer with a sensor that automatically controls the drying operation. Essentially, when a sensor dry mode is selected, the user places wet articles inside the dryer drum and selects a desired final dryness level. Instead of forcing the user to guess as to how long the process should take, the machine stops when the desired dryness level is reached. For this purpose, the machine includes at least one moisture sensor for detecting the level of moisture in the articles. The machine simply operates until the moisture sensor detects the final desired dryness level selected. By terminating the process upon achieving the desired final dryness level, there is no need to re-start the drying process to finish incomplete drying. In addition, extra energy is not expended to dry the articles beyond the desired dryness level.
Even with the sensor-dry mode, a typical prior art dryer will have its electrical heating element initially set to full power, again approximately 5,000W. Once again the temperature within the dryer will then rise until it reaches a preset maximum. The dryer will then enter a no power mode during which the heating element will be set to zero power. In a manner similar to the time-dry mode, the dryer will cycle between the heating mode and no power mode until the moisture sensor detects the final desired dryness level selected. Even with the use of a moisture sensor, articles of clothing placed within the drying machine are still subject to a temperature which varies from a high-temperature that may damage some types of clothing, to a low-temperature which is inefficient in that it will not properly dry the clothing in a reasonable amount of time.
Other known dryer arrangements work in a somewhat different manner. For example, U.S. Pat. No. 3,612,500 teaches controlling a first source of heat to establish a high output level for an initial portion of a drying cycle and a second source of heat to establish a lower output level during a subsequent portion of the drying cycle. Specifically, two heater elements are provided, rated at 3,100 and 2,500 watts respectively. During an initial portion of the drying cycle, both heaters are on. However, after the temperature in the drum reaches 160xc2x0 F., both heaters are turned off and, for the rest of the cycle, only one of the heaters is turned on and off, with a thermostat being used to control the dryer temperature. Even with this modification, the lower power level for the heater equals zero and the upper power level can only be set by the controller to either 5,600 watts or 2,500 watts.
In a similar manner, U.S. Pat. No. 3,508,340 discloses a dryer that provides heating at two power levels. During a first phase of heating, a high power of 4,400 watts is achieved by applying 240 volts to a heating element, while a low power of 1,100 watts is achieved later in the cycle by providing 120 volts to the heating element. Even with this teaching, the power supplied, while the thermostat is cycling, is zero and the upper power can only be set by the controller to either 4,400 watts or 1,100 watts.
Finally, U.S. Pat. No. 2,851,790 also discloses a temperature control system for a dryer. This patent teaches using a variable resistor in series with a bias heater so as to allow for variable adjustment of the output of the bias heater. The bias heater is used to heat a temperature control thermostat so that the thermostat will trip at a lower temperature. Regardless, the main heater still operates at either a high power level or at a no power level.
Based on the above, there exists a need in the art to provide a control system for a clothes dryer which allows for adjustment of the amount of power sent to a heating element of the dryer between various preset levels between full power and no power during a drying operation.
The present invention is particularly directed to a control system for a clothes dryer including a timer, a temperature sensor, and a circuit which is able to set the amount of power sent to a main heating element of the dryer during various cycles of operation. In accordance with the invention, the circuit automatically controls the maximum temperature reached in the dryer cycle. More specifically, a thermostat is employed to switch a main heater from a full power setting to a low power setting when the temperature in the dryer reaches a certain maximum preset temperature. Adding heat with a bias heater arranged near the thermostat during certain drying cycles changes the actual maximum temperature that the thermostat reacts to. The low power setting for the main heater is provided by switching one set of heating element contacts to neutral rather than turning the heating element completely off by disconnecting it from the power source. The switch returns the heating element to a full power setting by connecting the heating element contacts to different voltage sources when the sensed temperature within the dryer reaches a certain low temperature set point. This feature preferably only remains active during heat cycles, not during a cool down operation.
By providing low power, rather than no power, as the thermostat trips at its preset high temperature, the total drying time required to dry articles of clothing within the dryer is reduced. In any event, additional objects, features and advantages of the invention will become more readily apparent from the following detailed description of preferred embodiments thereof, when taken in conjunction with drawings wherein like reference numerals refer to corresponding parts in the several views.